The present invention relates generally to separately excited direct current (d-c) electric motors, and it relates more particularly to means for detecting a malfunction of the controls for such a motor.
In controlling an adjustable speed, separately excited d-c motor, it is common practice to increase or to decrease the magnitude of field current so that it varies with the magnitude of current in the armature of the motor according to a predetermined normal relationship. In the abnormal event of a malfunction or a fault in the control system, one or both of these currents can deviate appreciably from normal. If the faulty condition continues undetected for an extended period of time, the motor can suffer grave damage.
Persons skilled in the motor protection art are familiar with circuits and apparatus that disconnect or deenergize the motor upon detecting an overcurrent and/or overheated condition. Typically, such protective circuits respond to short circuits substantially instantaneously, but their response to lesser overload conditions is appropriately delayed. See for example U.S. Pat. Nos. 3,558,978 and 3,612,950. None of the pertinent prior art of which I am now aware is adequate to detect the symptoms of all possible faults in the controls of a separately excited d-c motor.